1. Field of the Invention
This invention relates to a pull bar, and more particularly to a tool for use in straightening indentations in sheet metal walls or skins especially when limited or no access is available to the back side of the indentation, such as in automobile bodies or fenders.
2. Description of the Prior Art
Various types of tools have been used in an attempt to return indented sections of sheet metal, such as in an automobile fender, to original shape. One known type of such a tool generally includes an elongate shaft having a sheet metal or self-tapping screw attachable to one end thereof, a weighted sleeve slidable over the shaft, and a stop at the opposite end of the shaft. In use, the sheet metal screw is threaded into the damaged portion of the automobile fender or the like and then the weighted sleeve is slid along the shaft to impact against the stop to thereby impart an outwardly directed force to the indented sheet metal. Examples of this impact type tool are disclosed by Guyton U.S. Pat. No. 2,791,926; Chartier U.S. Pat. No. 3,878,709; and Morgan U.S. Pat. No. 4,034,594. Chartier also discloses replacing the rigid shaft with a flexible wire cable. However, the basic operation of the tool is similar to that described above.
In another type of dent straightening tool, such as disclosed by Jones U.S. Pat. No. 3,977,230, a backing member is placed in contact with the outward surface of the dented area of the sheet metal. A clearance hole is provided in the backing member through which a sheet metal screw can pass to threadably engage with the dented area of the sheet metal. Means are provided to pull outwardly on the sheet metal screw thereby simultaneously pushing inwardly on the backing plate to thus force the indented sheet metal outwardly against the inner surface of the backing plate. An obvious drawback of this particular dent straightening tool, as well as those described above, stems from the fact that all of these tools require the use of a standard sheet metal or self-tapping screw to engage with the indented sheet metal. Commonly available sizes of sheet metal and self-tapping screws have thread diameters which are substantially smaller than the indented areas of, for instance, most automobile dents. Thus, the process of threading the sheet metal screw into the indented area of the automobile body and then pulling outwardly on the sheet metal screw as described above may have to be repeated many times resulting in a slow, expensive procedure.
Furthermore, the thread diameter of standard sheet metal screws is not substantially larger than the corresponding root diameter of the screw, and thus the thread is often not capable of withstanding the impact load imparted on it, especially when used in conjunction with the shaft and sliding weight type of tool. As a result, the sheet metal screw often is simply disengaged from or jerked out from the indented sheet metal when the sliding weight impacts against the stop at the end of the shaft of the impact type tool.
Also, although the thread of a sheet metal screw is initially sharp, such thread is often not substantially harder than the material in which it is being threaded into. Thus, the thread often becomes dulled so rapidly that it is difficult to engage it into the dented sheet metal.
Moreover, many sheet metal structures, especially in automobile bodies, are constructed with an outer wall and a inner wall spaced inwardly of the outer wall. When the outer wall is indented, it is pushed inwardly to a location closely adjacent to or even actually in contact with the inner wall, with the result that when the sheet metal screw is threaded into the outer wall it often also pierces and engages with the inner wall. Thus, it is usually very difficult to return the outer wall to its original shape since the inner wall is also pulled outwardly with the outer wall.